1. Field of the Invention
The present invention relates to a frost sensor for a domestic appliance having refrigerating equipment and, more particularly, to a new and improved frost sensor for such an appliance, the frost sensor having a fiber optic cable positioned about evaporator coils of the refrigerating equipment to determine the extent to which ice or frost has accumulated on the evaporator coils.
2. Description of the Prior Art
Appliances such as a refrigerator/freezer, a freezer or a heat pump typically include evaporator coils as part of their refrigerating equipment. In the case of a refrigerator/freezer, the evaporator coils may be disposed in a rear cavity behind a panel in one of the refrigerated compartments. A refrigerating medium circulating through the evaporator coil is typically cooled by a compressor to cool the air in the rear cavity of the refrigerator/freezer. The cooled air is then circulated through the refrigerated compartments to maintain those compartments at the proper refrigerated temperatures. Ice and frost tend to accumulate on the evaporator coils as the air is being cooled. A layer of ice and frost on the evaporator coils insulates the evaporator coils from the air to be cooled, making it difficult for the refrigerating equipment to maintain the refrigerated compartments at the desired cool temperatures.
In order to melt the ice or frost that has accumulated on the evaporator coils, a defrost cycle is typically initiated during which the compressor is turned off. In known refrigerator/freezers, the defrost cycle is controlled by a clock mechanism that turns off the compressor during preselected time intervals. During the defrost cycle, the clock mechanism also actuates a heating device located near the evaporator coils to assist in melting the accumulated ice and frost. However, depending on such factors as the ambient humidity and temperature, ice and frost accumulates on the evaporator coils at varying rates. Consequently, the clock mechanism may actuate the defrost cycle more often than is necessary to maintain the evaporator coils free of ice and frost.
In those systems that turn off the compressor at regular intervals, the compressor tends to be turned off for an unnecessary length of time in order to ensure that a sufficient amount of time is allowed to melt the ice or frost. When the compressor is turned off for an unnecessarily long period of time, the temperature in the refrigerated spaces rises resulting in the refrigerating equipment being run for an increased amount of time to return the refrigerated spaces to the desired cool temperatures after the defrost cycle has been completed.
In other known defrost systems, temperature sensors measure the temperature of the refrigerating medium and a defrost cycle is initiated in response to a predetermined change in that temperature. In order for these temperature sensors to be useful, they must be extremely sensitive to small changes in the temperature of the refrigerating medium because very small variations in this temperature occur over large variations in the accumulation of ice or frost on the evaporator coils. Consequently, these temperature sensors typically do not accurately reflect the amount of ice and frost that has accumulated on the evaporator coils.
In some defrost systems, optical devices have been used to determine the accumulation of ice and frost in a refrigerator. For example, in U.S. Pat. No. 2,225,932, a pair of Lucite rods positioned end to end are spaced apart in a refrigerator so that a gap is formed between the rods. Whenever the door of the refrigerator is opened, a bulb located at one end of one of the rods is energized. If the end of the other rod glows, it provides a visual indication to the user that frost has not built up in the gap between the rods to the extent that light from the bulb is blocked so that the refrigerator does not need defrosting. However, if too much frost has accumulated in the gap between the rods, the end of the other rod does not glow indicating that the refrigerator does need to be defrosted.
U.S. Pat. No. 4,578,959 discloses another optical system used in determining to what extent frost has accumulated on the evaporator coils of refrigeration equipment. In the system disclosed in that patent, electromagnetic radiation having a narrow band of wavelengths is projected onto the frost accumulating on the evaporator coils. The electromagnetic radiation passes through the frost to the evaporator coils where it is reflected back through the frost to a detector that is sensitive to the selected wavelength of the electromagnetic radiation. The amount of radiation that is detected is an indication of the amount of frost that has accumulated on the evaporator coils because the radiation is absorbed or scattered by the frost in a relation proportional to the thickness of the frost that has formed on the evaporator coils.
Neither of these patents (U.S. Pat. Nos. 2,225,932 and 4,578,959) discloses the use of fiber optics to detect the accumulation of ice and frost on evaporator coils of refrigeration equipment. On the other hand, fiber optics have been used to detect the present or absence of a liquid such as oil leaking from a storage tank. One such use is described in U.S. Pat. No. 4,270,049 wherein oil leaks drop onto a light guide or optical fiber. The amount of light traveling from one end of the optical fiber to the other end is affected by the amount of oil that has accumulated on the optical fiber to provide an indication of the amount of oil that has leaked.
Light transmitting tubes or the like have been used to detect the level to which a liquid has risen in a container or vessel. For example, such level detection devices are disclosed in U.S. Pat. Nos. 3,192,392; 3,683,196; 3,995,169; and 4,187,025. In U.S. Pat. Nos. 3,192,392 and 3,683,196, light transmitting tubes with at least one notch formed in their outer surfaces extend into a container of liquid. When light travels along the tube, it is reflected by the inclined surfaces of the notches. The amount of light transmitted through the tube varies depending on whether the liquid in the container has reached the level of the notches in the tube since light is reflected differently when the notches are filled with a liquid. In U.S. Pat. No. 3,995,169, light tubes are disposed within a tank of liquid. Each of the tubes has a U-shaped bend with flat surfaces along the outer sides of the bend. The amount of light that is transmitted through the light tube is altered by the extent to which the liquid makes contact with the flat outer surfaces of the bend. Another liquid level detector is disclosed in U.S. Pat. No. 4,187,025 wherein an elongated light conducting body or tube is formed with several curved sections. When the liquid rises to cover the curved sections of the tube, the amount of light that is transmitted from one end of the tube to the other end of the tube is altered.
Liquid level detectors are not the only devices in which light transmitting tubes having notches in their outer surfaces have been used. For example, in U.S. Pat. No. 2,909,857, a light pipe is disclosed which has a series of notches so that the light traveling through the light pipe illuminates counters positioned adjacent to each of the notches. In U.S. Pat. No. 3,526,880, notches in light conducting rods or fibers are positioned adjacent to sensors used in a binary digital memory. On the other hand, a light conducting rod formed of an array of stepped individual mirrors is disclosed in U.S. Pat. No. 4,196,962 for use in transmitting a laser beam.